Indoor Options to reduce temp with exhaust system: more CFMs or double the duct?

Mar 13, 2017
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I'm getting ramped up with my first grow and trying to dial in the environment temps to get me thru the summer.

Indoor grow in a detached garage with an 8x8 tent. I have two 600 watt lights in cool tubes and a charcoal filter all inline as the exhaust system. The exhaust fan is 8 inch, the ducting and tubes are all 6 inch. I also have a 4 inch centrifugal bringing in fresh air to the tent from the garage floor.

At first I was exhausting directly in to the garage right outside the tent and passive air intake. Then I added the 4 inch fan on the input. But since I was exhausting in to the garage and basically pulling in that warm air back in to the tent, the tent temp was too hot in my summer conditions.

The garage is insulated. And it has a storage loft/attic above the floor. And the roofing has 2 passive square vents.

I added more ducting for the exhaust, making the length longer, and now have the exhaust system going directly out of the garage roof instead of inside the garage. This is 6 inch ducting connected directly to the a vent in the roof. The vent is a square thing (not sure what to call it). It is not one of the round spinning whirly vents.

Right now, even on a moderate summer day, I'm getting tent temps of 30C even with all the active air movement. I don't have any temp data from our peak hot days yet, but it will be even hotter. I would like to be able to reduce the temp by a good 5 degrees C.

There is another vent in the roof. One option is to put a Y splitter in the ducting and exhaust to the second roof vent. I'm assuming if I can double the volume or capacity of the exhaust output vents with the Y I'll reduce the tent temps. The ducting would still be all 6 inch, but I'd exhaust to 2 exits instead of 1.

Or would I get better results using the same setup with one exit point but adding more CFM to the line?

My gut says having 2 outside external exhaust exit points will be the most effective option. Those square vents in the roof have to be restricting airflow compared to the spinners. And if I kept things the same and added another inline fan and additional CFM's as is, I would not see any improvement because of the vent size bottleneck.

Or would adding another booster inline fan to the existing exhaust be a better approach?

Or would adding an AC cooler to the garage itself be better?

I'm curious how all these things factor in reducing tent temps. Where do you get the most efficiency? Input or output CFMs? Size of ducting? Size of exit points? Air conditioning?

I should mention I just realized the temp test I did today were not done with any tent circulation fans. I have a couple of fans in the tent that I'll use during the grow, but are off right now. Would that make a 5 degree difference?
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I turned on my big fan in the tent hoping it would reduce temp but it did not. I read on this site a thread that has someone say a circulation fan will reduce room/tent temperature.

So I'm back to where I started.

Anyone got any thoughts on best options to look at?
I'm not about to do a whole bunch of math and stuff...
but i'll tell you this.. its only ever gonna get as cold as the air it takes in. <--- well close unless u got a turbine tunnel goin on
you could have an open roof but if the intake is 30' ..... it'll be hotter (lights and stuff)
I think I made a post on here with some video for a DIY ghetto ac

I agree very much with the last post on it, it will jack up humidity , so if its flower i'd recommend a proper a/c unit
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I added another 4 inch inline fan with 140 CFMs for intake air. Now I have 280 CFM's input.
The exhaust 8 inch inline fan is over 300 CFM's. But given the length of the ducting and a couple of bends I'm not sure I'm getting negative pressure now.

Anyway, with this change by adding more air in, I'm not reducing temps in the tent.

Guess my next option or approach is to find a way to exhaust more air out like I mentioned above. Going to get a Y/Wye splitter and insert it in to the exhaust duct line in the attic and run them out to the 2 vents in the roof, hopefully doubling the ability to exhaust air. And if that still doesn't do the trick, I'll move the second 4 inch inline from the input to the exhaust line to give me more CFM's on top of the two exhaust exit points.

Guess nobody can answer my questions because there are too many variables in each environment and it's really up to me and trial and error until I find the solution.
if you can afford to run AC than run AC!
I had a similar problem in an identical tent .. I had 800 m3/h carbon exhaust cooling the tubes but I had to add another 420m3/h with a carbon to suck the air out..and it was still 30-32 although outside was like 36.and the plant survived.
definitely, there was underpressure
Trying to avoid an ac unit. Biggest issue is there is no vent in the main garage for the ac exhaust to outside. I would have to run a long duct from the ac unit to the roof. I am not sure those portable ac units work well with long exhaust duct runs. And there is the cost of one to buy and run.

But if i cant fix this with more text exhaust capacity or other ventilation hacks i will have to look at ac.

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where is everyone who wants to talk about heat? I'm sure there must be tons of people dealing with heat this time of year. There are lots of old posts so maybe it's all done to death?

I built a "swamp cooler" in the last day or what some call all cheap DIY air conditioner. Took an old coleman cooler and cut out 2 holes on top. One for an inline fan to push air in, one for a duct tube out to run in to the tent for my fresh air in. I loaded up about 8 liters of regular ice from water (no salt) in to bottles and froze it all and put it in the cooler.

The idea is definitely promising. You can get some pretty cold air in that cooler and pushing that in to the tent is nice and helped reduce tent temperature. But the ice melts so fast which I was surprised by. Must be all that air coming in to the cooler.

I also loaded up a new cooler with normal ice cubes that I better insulated and one I don't want to cut up for a project. I did not use it for the tent. Just measure the ambient temp of the cooler and it was better cooler in temp than the first cooler.

So I dumped in all the cubes in to cooler 1, the DIY AC cooler, and it started out great but within minutes I could see the temp rising and the ice melting fast. This is just not a solution that will give me a full day of cool air with bottled ice.

After lots of different google searches I found out about people that are taking the swamp cooler hack and upping the game. The idea is to use water cooling thru copper coils or heater cores from engines. Create an ice batch and circulate that cold water thru a copper chiller like maybe a beer making wort chiller or a automotive heater core or an AC condenser. And find a way to run air over the copper chiller and then out the vent. This approach could hopefully make ice last longer since the ice is not in direct contact with the fan air.

I'm going to try a science experiment with this idea this weekend and see what I can make happen with it.

Part of the problem or dilemma for me is I only get about 4 weeks of too hot weather and an AC unit isn't really cost justified. And getting an AC unit setup in a garage is challenging too. So other approaches like exhaust tweaking, DIY cooler and ice, more CFM's, CO2, are attractive.

Here are a couple of links to the swamp cooler/diy AC with copper chiller/heater cores.

Do 18/6 light/dark. Make the hottest part of the day (noon to 6PM) your night (lights off). Reduce lights (mine have a 50% option. Cooler lights (LED or Fluorescent). Avoid summer grows---do spring and fall. Insulate the grow room from outside heat. Insulate attic roof. Mine have survived 115F temps with no problem, but I try to keep it under 95. 85 is best.
I am dealing with heat, but so far so good. I have a portable A/C unit set at 72 F in our 8' x 8' grow room. Right now the outside temps get up to mid 90s every day, and the grow room can get up to 92. I've got 1,000 actual watts of Mars Hydro LED, and the A/C unit can't keep up during the summer. I have 2 grow cycles - January thru May and August thru December. We shut down during June and July, too hot. Right now I put ice cubes in the grow pots, they melt slowly and help to lower the temps of the soil and roots. My plants are all looking good but they are only around 2 weeks into this grow cycle.
BTW, my 5 month cycle goes like this - plant 4 autos and 2 photos. 18/6 light cycle until autos are done, then 12/12 for the 2 photos. We discovered this by accident, but it sure has worked out nicely.